Care of Newly Planted Rhododendrons
So critical is the care of newly planted rhododendrons that the ARS publication The Fundamentals of Rhododendron and Azalea Culture devotes a whole section to the issue, which is reprinted here.
Before planting, dry root balls should be thoroughly soaked in a tub of water. Under normal conditions, it is not necessary to break apart a soil root ball; however, some loosening of the outer roots should be done to get the fine roots out of the existing root ball and into the new soil. This is particularly true if the plant was previously grown in heavy soil. With container grown plants, it is important to loosen and even cut some of the outer roots, especially if the plant is root bound. Cutting the roots encourages new roots to grow out into the soil. Hosing off the outer part of the soil or planting mix can be helpful in loosening roots for growth into their new planting hole.
Although rhododendrons will not tolerate stagnant soil moisture, they are shallow rooted and the roots may dry out during the summer even though deeper rooted plants show no sign of drought stress. Therefore, rhododendrons should be well watered during the summer, especially the first year after planting when the roots have not yet gotten out of the original root ball and into the surrounding soil. Rhododendron roots are very fine and it takes them longer to grow out into the surrounding soil than most other kinds of plants. Because of this, the newly planted rhododendron will get its water out of its original root ball, and if this ball is allowed to dry out it is very difficult to get it wet again. The soil around the planting hole can be wet, yet the plant root ball itself can be bone dry. This is an important point to remember Often the only way to re-wet a dry root ball is to place a dripping hose at the base of the trunk of the plant and let it run for several hours. Unless you have a very wet climate, doing this weekly in addition to the regular watering the plant gets during the first growing season will help get the plant off to a good start.
Some foliage droop is normal in dry weather, especially on warm afternoons, but when leaves still show signs of drooping in early morning, the plants are showing a need for water and should receive a good soaking. When air temperatures go above 95°F (or even lower for alpine types), rhododendrons and azaleas appreciate a misting to prevent desiccation of their foliage. In cold climates, watering or misting of foliage during warm days in the spring or on windy days when the roots are still frozen will help to keep rhododendrons in good condition.
A year-round mulch of some type of organic matter is desirable to conserve moisture and eliminate the need for cultivation. Because of their shallow roots, little or no cultivation should be done around rhododendrons. Weeds should be carefully pulled, or in extreme cases shaved off with a sharp hoe. A fairly deep mulch of leaves, pine needles, chips, bark or other organic material will practically eliminate weed growth. (Peat moss should not be used as a mulch because it sheds water when it dries out.) The coarser the mulch the better, as water and air are admitted while the mulch still retards evaporation by providing shade and reducing wind velocity over the roots. A mulch also helps to reduce temperature extremes in the root area.